(1) Field of the Invention
The present invention relates to a copper powder for use in a conductive paste which allows the paste to possess excellent initial electrical conductivity, thermal resistance, and humidity resistance, and which enables the paste to maintain stable electrical conductivity for a long period of time.
(2) Description of Prior Art
In general, the electrical conductivity of a copper powder tends to drop because the surface of the copper powder is oxidized. Therefore, in order to obtain a copper powder with an adequate degree of conductivity when it is mixed with a synthetic resin, various proposals have been made with respect to methods of surface treating the copper powder or methods which employ additives.
An example of a method of surface treating a copper powder is one which employs a protective layer made of benzotriazole, tryltriazole, chromate, or silicate. Although it can be appreciated that such a protective layer has the effect of preventing discoloration, the layer greatly impairs conductivity and is thus unsuitable for a use in which electrical conductivity is aimed for. U.S. Pat. No. 4,305,847 discloses a method of adding an organic titanic compound to a conductive coating or paste.
However, such a method of adding various additives gives no consideration to the oxidized layer on the surface of a copper powder, which will greatly affect the level of conductivity of the coating. As a result, even when a large amount of copper powder is blended, the method fails to achieve a certain level of conductivity which has a specific resistance of 10.sup.-4 .OMEGA..cm approximately equal to those of silver conductive coatings.
In addition, since the method provides no anti-oxidization treatment of the copper powder, the surface of the copper powder is oxidized and the conductivity is degraded within a short period when the resulting coating film is subjected to environmental reliability tests. Thus, this coating shows low reliability.
In view of advances being made in the development of printed circuits there is a demand for a copper powder which may be used in a conductive paste applicable to printed circuits. Such a copper powder should possess a sufficient degree of thermal resistance to withstand the high temperature at which the resin used in the paste is cured and also the high temperature during soldering process. However, no such copper powder has yet been found.